Tree density influences ecohydrological drivers of plant–water relations in a larch boreal forest in Siberia

Abstract

Transpiration and stomatal conductance in deciduous needleleaf boreal forests of northern Siberia can be highly sensitive to water stress, permafrost thaw, and atmospheric dryness. Additionally, north‐eastern Siberian boreal forests are fire‐driven, and larch (Larix spp.) are the sole tree species. We examined differences in tree water use, stand characteristics, and stomatal responses to environmental drivers between high and low tree density stands that burned 76 years ago in north‐eastern Siberia. Our results provide process‐level insight to climate feedbacks related to boreal forest productivity, water cycles, and permafrost across Arctic regions. The high density stand had shallower permafrost thaw depths and deeper moss layers than the low density stand. Rooting depths and shallow root biomass were similar between stands. Daily transpiration was higher on average in the high‐density stand 0.12 L m−2 day−1 (SE: 0.004) compared with the low density stand 0.10 L m−2 day−1 (SE: 0.001) throughout the abnormally wet summer of 2016. Transpiration rates tended to be similar at both stands during the dry period in 2017 in both stands of 0.10 L m−2 day−1 (SE: 0.002). The timing of precipitation impacted stomatal responses to environmental drivers, and the high density stand was more dependent on antecedent precipitation that occurred over longer periods in the past compared with the low density stand. Post‐fire tree density differences in plant–water relations may lead to different trajectories in plant mortality, water stress, and ecosystem water cycles across Siberian landscapes.

Root‐associated fungi not tree density influences stand nitrogen dynamics at the larch forest–tundra ecotone

“…We observed that roots excised from the organic horizon were most active in N uptake. This supports findings from nearby stands that demonstrated that larch water uptake was from shallow sources associated with precipitation more so than deep water sources associated with thawing permafrost (Kropp et al, 2019). Analysis of the root traits of a congeneric, L. decidua , revealed that larch have a conservative rather than acquisitive strategy, for example, high diameter, low specific root length and high tissue density (Liese et al, 2017) similar to other conifers (Withington et al, 2006).…”

“…To assess relationships between tree density and characteristics of larch roots and root-associated fungi important to N acquisition, we intensively sampled 10 of 26 larch stands across a tree density gradient where tree productivity (Walker et al, 2021), understorey composition (Paulson et al, 2021) and ecohydrology (Kropp et al, 2019) have been described. Tree densities at the 10 stands (each stand ~0.5 ha, distance between stands 0.1 and 3.0 km) ranged from 0.10 to 3.70 trees/m 2 representing the range of densities that occur across the full density gradient (0.03 to 3.70 trees/m 2 ).…”

Recent atmospheric drying in Siberia is not unprecedented over the last 1,500 years

“…Our findings suggest a trend towards higher simulated VPD (almost 10%) across the entire period (516-2004 CE) compared to the global average of 1% 14 . The drastic thawing of permafrost in the subarctic forest and reduction of precipitation with increasing evaporative demand 4,13,44 could increase the risk of forest decline and mortality at these sites in the (near) future.…”

“…Precipitation and permafrost-thawed water during the summer are crucial for trees growing in regions with severe temperature limitations and predominantly continental climate conditions 1 – 4 . Although precipitation is low in the Siberian North, due to low temperatures, water loss is not yet as large as observed in European forest ecosystems 5 .…”

Siberian Ecosystems as Drivers of Cryospheric Climate Feedbacks in the Terrestrial Arctic

“…Similarly, forest impacts on surface energy exchange exert a series of interrelated controls on permafrost and snow thermal dynamics (Loranty et al, 2018;Stuenzi et al, 2021). Variation in forest recovery within a single fire perimeter near Cherskiy has resulted in even-aged stands with canopy cover ranging from ∼10-95% (Paulson et al, 2021), and winter albedos of ∼0.3 and ∼0.7 at representative highand low-density stands (Kropp and Loranty, 2018;Kropp et al, 2019). Corresponding to post-fire tree density, summer and winter soil temperatures are warmer at sites with lower tree cover (Figure 1B) as a result of lower canopy shading and insulation from organic soils, and snow redistribution, respectively.…”